Donald Trump and Vladimir Putin have a lot in common—elephantine egos, jowls, a post-fact approach to the world. They also happen to run two of the world’s most unequal countries, as major economies go.

The chart below shows the share of national income in three countries—the US, Russia, and France—claimed by the top 10% of earners since the early 1900s. (Income counts as wages, interest, rent, dividends, realized capital gains, pensions, and unemployment benefits before taxes and...